Light strings having incandescent lights connected electrically in a series are well known, especially around the holidays when such light strings are used for decorative purposes. More recently, the use of light emitting diodes (LEDs) in place of incandescent lights has become more prevalent. Early versions of LED-based decorative light strings relied upon bulky, external power transformers to convert readily available alternating current (AC) power to direct current (DC) power. The LEDs were typically wired in parallel and with the appropriate DC voltage applied across each LED. Eventually, series connected decorative LED strings that operated directly on AC power became available.
Although these AC-powered LED strings provide desirable characteristics such as high reliability, long life, and low energy consumption, these strings often exhibit a “flickering” effect. This “flicker” results from the LEDs being operated on sinusoidal AC power. As the AC voltage alternates positive and negative, each LED turns on and off with the changing supply voltage. The result is a visible flickering of the LEDs.
FIG. 1 depicts an example of a typical prior art LED string as disclosed in U.S. Pat. No. 6,461,019 issued to Allen. Allen discloses an LED string with one or more series of LEDs connected directly to an AC power source P. In operation, LED series A, comprising LED 1A through LED NA is lit for approximately one half of the sinusoidal power cycle, while LED Series B, comprising LED 1B through NB is lit during the other half of the power cycle. As such, the LEDs of Series A and B alternately emit light, causing a generally noticeable flicker effect.
Some light strings that operate on AC power attempted to solve the flicker problem through full-wave rectification applied to all the LEDs. This typically mean using a rectifying bridge located in an external enclosure, or in the power plug. FIG. 2 depicts a prior art LED string that utilizes a full-wave bridge rectifier to provide full-wave rectified AC, considered “DC,” power to all of the LEDs in the light string. Although flicker can be reduced significantly through such a circuit design, an LED string implementing such a circuit typically requires the use of an external enclosure to house the bridge rectifier. Adding an external enclosure to the LED string adds additional cost and complexity to the LED set, detracts from the decorative value of the light string, and may eliminate certain size-sensitive applications. Further, if such an LED string is to include an end connector for connecting a second string to the end of the first string, the end connector must be directly wired to the plug, requiring more wire than standard light strings.
Alternatively, the bridge rectifier may be added to the power plug. U.S. Pat. No. 5,777,868 issued to Gibboney, Jr., discloses a power plug that includes a built-in bridge rectifier. Using such a power plug with a decorative light string adds significant cost and complexity to what are typically relatively simple devices. In addition to the additional cost, which is partially due to the use of non-standard components, such a plug may not allow the use of current taps which facilitate the stacking of power plugs so that several light strings may be plugged into the same power source.
Another known alternative is to split the bridge, locating one pair of rectifying diodes in the power plug, and one pair in the end connector. Such an LED string is depicted in FIG. 3, and disclosed in U.S. Pat. No. 6,972,528, issued to Shao.
As depicted in FIG. 3, Shao discloses the anodes of a pair of rectifying diodes connected to multiple series-connected LEDs, which in turn are connected to a filtering circuit and another pair of rectifying LEDs. In this embodiment, full-wave rectified AC power, or essentially DC power, is provided to all LEDs. The pair of rectifying diodes nearest the power plug are packaged in the plug, while the other pair of rectifying diodes are packaged with the end connector, or “rear plug”. Alternatively, they may be located in their own larger external housing. Although the flicker may be reduced using such methods, non-standard power plugs and end connectors or external housings need to be designed and manufactured.
The difficulties and drawbacks of these known decorative LED strings are overcome by the LED strings and methods of the present invention.